The biggest reason for attorneys hiring their own lawyers is because of the emotional distress involved in a personal case. Being accused of a crime can be stressful, and lawyers may have difficulty preparing their own defense as a result. Their emotions could also cause them to make decisions reactively rather than thinking things through.
An individual may not be able to bond out of jail while awaiting trial, depending on the offense. In this instance, it could be very difficult for someone who is behind bars to prepare his or her own legal defense due to a lack of resources. As such, it is usually better to let someone with access to witnesses and other evidence handle a criminal matter. Not only that, but it ensures that deadlines for filing motions can be met, which may not be possible without access to the courthouse.
Lawyers who have practiced for some time also tend to know who the best criminal defense attorneys are. As a result, they are in a much better position to hire the very best counsel than members of the general public are. Many times, attorneys choose to hire another lawyer to represent them simply because they know there are others out there that are more capable than they are.